Spark plugs



Dec. 17, 96@ l. szaL/AGY: 3p47927@ SPARK PLUGS Filed May 51 1967INVENTOR ATTORNEYS United States Patent O 3,417,276 SPARK PLUGS lstvanSzilagyi, 8 Nagymezo utca, Budapest 6, Hungary Contiuuation-in-part ofapplication Ser. No. 391,905,

Aug. 25, 1964. This application May 31, 1967, Ser. No.

Claims priority, application Hungary, Sept. 17, 1963,

1 Claim. (Cl. 313-143) ABSTRACT F THE DISCLOSURE A spark plug has amiddle electrode having a hot end and a cold end and a casing having acounter electrode at the hot end, and an insulating shell holding andsurrounding the middle electrode but leaving an interstice therearound.The shell is fixed to the casing by a bonding layer, along a conicalsurface that converges toward the cold end. This shell is a partcylindrical and part conical tubular body of constant wall thickness,and the middle electrode is fixed in the shell at the cold end. Theinterstice is unobstructed along its whole length, thereby to provide along creeping path between the electrodes. An insulating layer coversthe middle electrode up to its hot end.

Cross reference to related application This application is acontinuation-impart of copending application Ser. No. 391,905, filedAug. 25, 1964.

This invention is concerned with spark plugs of the type having a centeror middle electrode, a casing made of electrically conductive materialand provided with a counter electrode, and a shell made of insulatingmaterial and surrounding said middle electrode but leaving an intersticetherebetween.

Such spark plugs are already known and are called gas-insulated sparkplugs. They are distinguished by having no insulating component partswhich would glow at high temperatures, because the middle electrode ofthe spark plug is surrounded and insulated by exploding gas underpressure. The covering shell of insulating ymaterial is, as a rule, madeofglass by which it is rendered possible to observe the spark dischargebetween electrode and counter electrode as well as the course ofexplosion. The small amount of gas inside the shell is ignited upon thespark being discharged and ignites, in turn, the mixture present in theadjacent explosion chamber.

As is known, such gas-insulated spark plugs are distinguished by thegreat advantages of reliable ignition, considerable saving in fuel, arelatively higher engine output and performance, improved accelerationcondi tions, perfect controllability of ignition and carburetion, and asurprisingly easy readiness of starting. Furthermore, they permit theemployment of relatively inexpensive glass instead of expensivecorundum.

The main object of the present invention is to provide a Spark plug ofthe above-described type distinguished by a long lifetime and by greatreliability of operation. At the same time, the new spark plug is ofvery simple construction and inexpensive to produce. For this purpose,the invention is based on a spark plug having, in a manner known per se,a middle electrode defining a hot and a cold end, a casing provided witha counter electrode at said hot end, and an insulating shell holding andsurrounding said middle electrode but leaving an interstice therearoundand fixed in said casing along a conical surface converging toward saidcold end. The said shell is a tubular body of substantially invariablewall thickness fixed to said casing by means of a bonding layer, saidmiddle electrode being fixed in said shell at said cold ICC end so as toleave said interstice unobstructed along its whole length, and beingcovered by an insulating layer so as to provide a long creeping pathinside said interstice between said electrodes.

lt has been found that the employment of a bonding layer ensures auniform warming up of the wall of the insulating shell along its wholelength, besides compensating differences in thermal expansion betweenthe insulating shell and the metal casing. Thereby, also abrupt changesof the temperature of the shell where it emerges from the casing areavoided. This is vital for the long lifetime of the new spark plug.

If it is considered that the shell is not incandescent in operation,because of the gases by which its walls are constantly swept and cooled,the long creeping path obviously ensures a reliable operation of the newspark plug in which deposits will not be transformed into conductivecomponents short-circuiting the spark gap even if such deposits wereable to cling to the smooth glass wall, which they are not.

The simplicity of the construction follows from the small number ofcomponent parts and the fixing of their mutual positions by means ofbonding the glass shell in the casing and of soldering the middleelectrode in the glass shell.

Further details of the invention will be described by taking referenceto the accompanying drawing which shows, by way of example, alongitudinal sectional view of an exemplified embodiment of thegas-insulated spark plug according to the invention.

A middle electrode 10 of the spark plug according to the inventiondefines a hot end which is at its extremity adjacent to the spark gap ofthe plug, and a cold end which is opposite to the former. The middleelectrode 10 cooperates, in a manner known per se, with a counterelectrode 11 which is fixed in a casing 12 made of electricallyconductive material such as metal. The middle electrode 10 is surroundedby a shell 14 of unobstructed annular cross-sectional area made ofinsulating material, an interstice 13 being left therebetween.

The shell forms a tubular body 14 made of glass or the like, and hasunvarying wall thickness. It is cemented into a bore of the casing 12. Abonding layer is designated by reference character 15. The suitabilityof cementing for such purpose has amply been provided in the course ofexperiments. Viz, the outer threaded portion of the casing 12 of thespark plug shown in the drawing is, in all cases, connected with a seatwell cooled by air or water. Being made of highly heat-conductivematerial, it assumes the low temperature of its ambiency. On the otherhand, the glass body 14 covering the greater portion of the bore in thecasing 12 and being made of a material of relatively poor heatconductivity likewise protects the metal parts of the spark plug againstheat action of exploding gases. Thus, it is easy to find a bond themelting point of which suitably surpasses the maximum temperature towhich the layer 15 may be exposed. Most epoxy resins are suitable forthe purpose. They should withstand temperatures up to 350 F. and have abonding strength of at least about 1500 p.s.i. Suitable examples are theepoxy resins sold under the trade names Araldite and Devcon.

The casing 12, the shell or glass body 14 and the bonding layer 15 fitone another along partly conical and partly cylindrical surfaces. Theconical portion diverges toward the hot end of the spark plug andconverges toward the cold end thereof, that is, downwardly and upwardly,respectively, as regards the drawing, Obviously, the surface of the borein the casing 12 or the outer surface of the glass body 14 may begraduated or composed of surface portions associated with various coneangles as well.

The construction by which shell 14 is partly cylindrical and partlyconical is particularlyy advantageous. The conical portion facilitatesthe insertion and positioning of shell 14 in casing 12 duringmanufacture, and also ensures that the pressure in the cylinder willhelp retain the shell 14 assembled in casing 12. The cylindrical portionprovides enough cross-sectional area at the cold end to facilitate theattachment of middle electrode 1() at that end and also ensures theprovision of an interstice 13 whose shape promotes turbulence 0f the gasand hence cooling of the middle electrode 10. Finally, it is to be notedthat length of the cylindrical portion of shell 14 extends within casing12. The resulting uniformity of cross section of shell 14 where itemerges from the cold side of casing 12 provides protection againstdamage from steep temperature gradient at this point of emergence andresultant concentrations of mechanical stress.

Furthermore, the middle electrode 10 is covered by a layer 28 made of amaterial of low thermal capacity. Such layer may consist for example ofa thin-walled insulating pipe, whose thickness is exaggerated in thedrawing, of enamel or any sealing glass, whose coefficient of thermalexpansion lies between those of middle electrode 10 and shell 14. Due tothe low thermal capacity, the thermal variations occurring in anassociated internal combustion engine can easily be followed by theelec` trode 10 and its protecting layer 28. Moreover, the creeping pathof the spark plug, illustrated by dotted line 30, becomes doubled inlength, which is of particular significance in the case of heavy loads.The creeping path" is the shortest path by which an electrical dischargeor leakage could occur along a solid surface between electrodes 10 and11 instead of across the air gap between the electrodes.

From a consideration of the foregoing disclosure, therefore, it will beevident that the recited object of the present invention has beenachieved.

Although the present invention has been described and illustrated inconnection with a preferred embodiment, it is to be understood thatmodifications and variations may be resorted to without departing fromthe spirit of the invention, as those skilled in this art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the present invention as defined by theappended claims.

Having described my invention, I claim:

1. A spark plug having a middle electrode having a hot end and a coldend, a casing having a counter electrode at said hot end, an insulatingshell holding and surrounding said middle electrode but leaving aninterstice therearound, said shell being tixed in said casing along aconical surface converging toward said cold end, said shell being atubular body of substantially constant wall thi:kness said middleelectrode being fixed in said shell at said cold end so as to leave saidinterstice unobstructed along its whole length thereby to provide a longcreeping path inside said interstice between said electrodes, and aninsulating layer covering said middle electrode up to the hot end of theelectrode thereby to increase the length of said creeping path betweensaid electrodes.

References Cited UNITED STATES PATENTS 1,510,253 9/1924 Boehm 313--144OTHER REFERENCES 206,376 ll/l923 Great Britain.

JAMES W. LAWRENCE, Primary Examiner.

u D. J. GALVIN, R. F. HOSSFIELD, Assistant Examiners.

U.S. Cl. X.R. 313-141, 142

